This application claims priority of Korea patent Application No. 2000-000934, filed on Jan. 10, 2000.
(a) Field of the Invention
The present invention relates to a lithium ion battery, more particularly to a new electrolyte and a lithium ion battery which comprises the same, using an anode including graphitized carbon and a cathode including lithium-containing transition metal oxide.
(b) Description of the Related Art
Ever since the lithium ion liquid secondary battery was first commercialized by Sony Co., the lithium ion liquid battery has been used increasingly in portable computers and cellular phones etc., instead of the lithium ion secondary batteries of prior art, as it has a higher energy density. The lithium ion liquid secondary battery comprises an anode including carbonaceous material as an anode active material and a cathode including a metal oxide of LiCoO2 etc. as a cathode active material, and is prepared by intercalating a porous polyolefin-based separator between the anode and the cathode, then by injecting a non-aqueous electrolyte having a lithium salt of LiPF6 etc. When the battery charges, the lithium ions of the cathode active material are released and then are inserted into the carbon layer of the anode. When the battery discharges, the lithium ions of the carbon layer of the anode are released and then are inserted into the cathode active material. The non-aqueous electrolyte plays a mediating role moving the lithium ions between the anode and the cathode. The electrolyte should be stable within the scope of the operation voltage of the battery, and be able to transfer the ion with sufficiently fast velocity.
As an electrolyte, U.S. Pat. Nos. 5,521,027 and 5,525,443 disclose an admixture electrolyte of a linear carbonate and cyclic carbonate. The cyclic carbonate has a large polarity and thus is sufficiently capable of dissociating lithium, but has low ion conductivity due to the large viscosity. Therefore, in these patents, mixing linear carbonate with a low polarity and a low viscosity reduces the viscosity of the electrolyte comprising the cyclic carbonate.
The above cyclic carbonate includes carbonates such as ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), butylene carbonate (BC) etc. PC has a good low temperature performance due to a low freezing point of xe2x88x9249xc2x0 C. However, if an anode uses graphitized carbon of a large capacity, there is the problem of a sudden reaction between PC and the anode when the battery is charging. Thus, EC is commonly used, as it forms the most stable protecting film among the cyclic carbonates in a battery comprising an anode using graphitized carbon. However, if EC is used in a large amount, the low temperature performance of the electrolyte is abruptly deteriorated due to the EC""s high melting point of 37xc2x0 C. To resolve this problem, it is common to use a two-component electrolyte by mixing in a linear carbonate having a low melting point and a low viscosity as a second component with the EC.
The above linear carbonates include carbonates such as dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) etc. If EMC, which has the lowest melting point of the listed carbonates, namely xe2x88x9255xc2x0 C., is used, the battery exhibits excellent low temperature performance.
However, merely mixing cyclic carbonate and linear carbonate does not satisfy the need for a high capacity and low temperature performance in the lithium ion battery.
In fact, there has been research directed to improving the low temperature performance of an electrolyte comprising EC, by adding another electrolyte or new additives to the electrolyte of the prior art. The literature (J. Electrochem. Soc. 146(2), 485, 1999) discloses that an electrolyte of a three-component system prepared by mixing EC, diethyl carbonate (DEC), and dimethyl carbonate (DMC) has an excellent low temperature performance, better than the two-component system. Other literature (J. Fluorine Chem. 87 (1998) 221) discloses that low temperature performance is improved by adding CHF2COOCH3 to an electrolyte comprising EC and DEC.
If additives are added to the electrolyte as in the above methods, ion conductivity can be improved due to a lower freezing point and lower viscosity at low temperature, as the molecules of the electrolyte are prevented from forming a regular configuration with each other. That is if an electrolyte of more than the three components is prepared, wherein a suitable organic material as the third component is further added to the electrolyte of the two-component system, there is the effect of a freezing point depression when the electrolyte has a suitable composition, and also the effect of improving the charge performance at low temperature due to a reduction of viscosity at low temperature.
In fact, the electrolyte must be shown to be electrochemically stable within the scope of the operation voltage of the battery, and must be shown to have low reactivity with the graphitized carbon, and must not shorten the charge-discharge cyclic life by forming a stable protecting film on the anode. However, there has heretofore been no electrolyte which is known to satisfy the above condition.
It is an object of the present invention to provide a new compound that is electrochemically stable within the scope of the operation voltage of the battery, as the compound has both a cyclic carbonate structure and a linear carbonate structure in the molecule. The compound has a low reactivity with graphitized carbon of high electric capacity, and does not shorten a charge-discharge cyclic life by forming a stable protective film on an anode and thus can be used as a component of electrolyte.
It is other object to provide an electrolyte comprising the above new compound.
It is another object of the present invention to provide a lithium ion battery having a large electric capacity and a superior low-temperature performance comprising an anode including a graphitized carbon and a cathode including a lithium-containing transition metal oxide, a porous separator, and an electrolyte including a lithium salt and the above new compound.
In order to achieve the above objects and others, the present invention provides a compound shown in Formula 1: 
wherein, R is methyl or ethyl group.
The present invention also provides an electrolyte comprising:
a) a lithium salt; and
b) the following compound shown in Formula 1 comprising both a cyclic carbonate structure and a linear carbonate structure in molecule: 
wherein, R is methyl or ethyl group.
The present invention also provides a lithium ion battery comprising an anode including graphitized carbon as an active material, capable of absorbing and releasing lithium ions reversibly, a cathode including a lithium-containing transition metal oxide as an active material, capable of absorbing and releasing lithium ions reversibly, a porous separator, and an electrolyte, the electrolyte comprising:
a) a lithium salt; and
b) the compound shown in Formula 1 comprising both a cyclic carbonate structure and a linear carbonate structure in molecule: 
wherein, R is methyl or ethyl group.
The present invention is described in detail as follows.
The present invention relates to the compound shown in Formula 1 as an additive for an electrolyte of a lithium ion battery comprising an anode including a graphitized carbon, a cathode including a lithium-containing transition metal oxide, a porous separator, and a lithium salt-containing electrolyte.
Since the compound comprises both a cyclic carbonate structure and a linear carbonate structure in the molecule, the battery can possess a large capacity and improved low temperature performance when the compound is used for the lithium ion in a secondary battery including a lithium salt. In particular, the compound is electrochemically stable within scope of the operation of the battery, has low reactivity to graphitized carbon, and forms a stable protecting film due to a small possibility of insertion of the carbonaceous material of an anode together with the lithium ion, as the molecular size of the compound is relatively larger than those of EC and PC.
In the compound shown in Formula 1 comprising both a cyclic carbonate structure and a linear carbonate structure in the molecule, when R is methyl group, the compound is 4-carbomethoxymethyl-1,3-dioxolan-2-one; and when R is ethyl group, the compound is 4-carboethoxymethyl-1,3-dioxolan-2-one.
The compound shown in Formula 1 of the present invention may be prepared according to the following Scheme 1: sodium is immediately dissolved by adding in glycerol-1-allyether, then diethyl carbonate is added to obtain 4-allyloxymethyl-1,3-dioxolan-2-one as a first intermediate product, and a palladium carbon and p-toluene sulfonic acid are added to the first intermediate product. Then the mixture is reacted and distilled to obtain 4-hydroxymethyl-1,3-dioxolan-2-one as a second intermediate product, and the second intermediate product is reacted with methyl chloroformate or ethyl chloroformate, then extracted with methylene chloride solvent to obtain the compound shown in Formula 1. 
The compound shown in Formula 1 of the present invention may be used in an electrolyte of a lithium ion battery comprising only a lithium salt, or an electrolyte of a cyclic carbonate or a linear carbonate as well as a lithium salt. In particular, when the compound is used as a third component in an electrolyte comprising a cyclic carbonate and a linear carbonate in a lithium ion battery including graphitized carbon of high capacity, the lithium ion battery may exhibit a high capacity of graphitized carbon, a superior charge-discharge cyclic life and a superior low temperature performance by reducing irreversible capacity.
The electrolyte comprising the compound of the present invention is a non-aqueous solution containing a lithium salt. In particular, the lithium salt is preferably selected from the group consisting of LiClO4, LiCF3SO3, LiPF6, LiBF4, LiAsF6, and LiN (CF3SO)2. The electrolyte of the present invention may include an ester or a carbonate compound which is at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), -butyrolactone, sulfolane, methyl acetate (MA), methyl propionate (MP), and methyl formate (MF).
The present invention provides a lithium ion battery comprising an anode including high capacity graphitized carbon as an active material, capable of absorbing and releasing lithium ions reversibly, a cathode including lithium-containing transition metal oxide as an active material, capable of absorbing and releasing lithium ions reversibly, a porous separator, and an electrolyte containing a lithium salt and the compound shown in the above Formula 1.
To provide the lithium ion battery, the graphitized carbon employed has preferably a interplanar spacing (d002) of less than 0.338 nm as measured by X-ray diffraction of the carbonaceous material, and a specific surface area of less than 10 m2/g as measured by the Brunauer-Emmett-Teller (BET) method. The lithium-containing transition metal oxide is preferably selected from the group consisting of LiCoO2, LiNiO2, LiMn2O4, and LiNi1xe2x88x92xCoxO2 (wherein, 0 less than xxe2x89xa61). In particular, the above battery consists of an anode composed of an active carbonaceous material and polyvinylidene as a binder resin, a cathode composed of a lithium-containing transition metal oxide, a conductive agent, and polyvinylidene difluoride as a binder resin. The elements of the battery may be prepared by general methods. A lithium ion battery of a high capacity and a superior low temperature performance may be more easily prepared by using the compound shown in Formula 1 of the present invention in the electrolyte.